The present invention concerns a procedure for drying moving web material, in which on the material to be dried infrared radiation is directed and in which the moving web material is transported through the radiation zone of an infrared radiator, where the web material to be dried absorbs radiation.
The invention also concerns an IR dryer.
In paper and pulp industry, and in other branches of industry as well, moving web material is dried. Paper manufacturing and paper conversion include a number of steps in which it is necessary to accomplish drying by a non-contact method, that is using appropriate radiation and/or hot gases or air.
Existing types of infrared radiation apparatus used in drying web material consist of high-temperature quartz tube radiators or gas-operated medium wavelength radiators. The wavelength range of high-temperature short wave radiators is mainly 0.5 to 2 .mu.m, with peak at about 1.2 .mu.m. Shortwave radiation is penetrating when drying a thin web, because the coefficient of absorption of the material is poor as a rule in the wavelength range between 0.5 and 2.0 .mu.m, peak absorption usually occurring in the range above 3.0 .mu.m. As a consequence, the emission peak of the radiator and the absorption peak of the material do not coincide. However, high power per unit area is achieved with a high-temperature short wave radiator. The total power may be up to 450 kW/m.sup.2, in which case the radiant power absorbed in the web is more than 100 kW/m.sup.2. Power outputs of the magnitude are required when rapid drying is aimed at; this, in its turn, is required e.g. in the paper coating process. Short wave infrared radiators have also been employed under zone control in order to control the moisture profile of paper web material in the direction across the web.
The wavelength range of medium long wave infrared radiators is mainly 1.5 .mu.m to 6 .mu.m. The wavelength corresponding to maximum intensity is located at about 3.0 .mu.m. The same point is also usually one of the absorption points of the water constituting the object to be evaporated. At this point the cellulose fibres also display good absorptivity. Owing to the circumstances mentioned, the radiation efficiency of the radiation from a medium long wave radiator is high, about 45%, while this figure is about 25-30% for short wave infrared radiation apparatus, or for a high-temperature radiator, in cases in which thin web materials are being dried. The efficiency of both types of radiator increases with increasing material thickness.
The maximum radiation power achievable with medium wave infrared radiators is 60-75 kW/m.sup.2 when using a unilateral radiation source and 120-150 kW/m.sup.2 when using a radiation source on both sides.
The dryer built up with an infrared radiation means, or the IR dryer, is composed of a radiant surface which is located as close as possible to the surface to be dried. In apparatus of prior art, the radiant surface is encapsulated in a housing and the housing is installed, fixedly or provided with a motion mechanism, at a suitable location, attached to the frame structure of the process equipment. In said dryers the use of a counter-reflector is further known which throws back the radiation that has passed through the object being dried and thereby enhances the drying process. Furthermore, prior art knows air-conditioning systems employed in association with IR dryers, which serve the purpose of enhancing the drying and, at the same time, serve as coolers. The IR dryer may moreover comprise a system by which the drying power of the apparatus can be controlled.
Numerous different IR dryers used to dry a moving web, or a web material, are known in the art. Their operation is based on the ability of bodies to emit electromagnetic radiation, which is characteristic of the temperature which the body has. Another characteristic feature of the radiation is that, instead of one single wavelength, the radiator emits a plurality of wavelengths, whereby the characteristic emission spectrum of the particular radiator is created. Furthermore, according to the laws of physics, it is a characteristic feature of the radiation that when the temperature of the radiant body becomes higher, the radiant heat transfer to the target material increases in proportion to the fourth power of the differential temperature between the bodies.
However, the temperature of the radiator is not the exclusive factor determining how much radiation can be caused to be absorbed in the material that is being dried. The coefficient of absorption, which states the proportion of the radiation incident on the surface of the body that is absorbed by the material, is determined by the temperature, moisture content, thickness, material, surface roughness and lightness of colour of the body that is being dried. As a rule, however, the coefficient of absorption is a function of the wavelength in that in the short wave range the coefficient of absorption of a thin material is poorer than in the medium or long wave length.
On the basis of their maximum intensity wavelength, radiators are divided into short, medium and long wavelength radiators, the last-mentioned being rarely applied in technical processes. As IR radiation sources operating in the short wave infrared range those radiaters are counted which emit radiation having its maximum intensity wavelength in the wavelength range from 0.76 to 2.00 .mu.m. As IR radiation sources operating in the medium infrared range those radiaters are counted which emit radiation having its maximum intensity wavelength in the wavelength range from 2.00 to 4.00 .mu.m.
The temperature relationship is found with the aid of the Wien shift, from the formula: EQU .lambda..sub.max .times.T=2.8978.10.sup.-3 (mK)
The temperature range for a short wave radiator is found to be 3540.degree. C. to 1176.degree. C. and that for a medium wave radiator, 1176.degree. C. to 450.degree. C.
IR dryers operating in the short wave range are nowadays almost exclusively electrically driven. In them a tungsten wire, usually placed within a quartz tube, is made incandescent with the aid of electric current. In order to counteract oxidation, the tube is filled with an inert or halogen gas. The filament temperature is usually about 2200.degree. C., whereby the wavelength corresponding to maximum radiation intensity is about 1.2 .mu.m. However, the surface structure of the lamp cannot tolerate temperatures in excess of 300.degree. C., for which reason the lamps have to be cooled by blowing cool air past between the lamps. The air is discharged through holes in the radiant surface into the drying zone. As it passes through between the hot lamps, the air is heated several ten degrees Centigrade, and when discharging into the drying zone this air possesses a fairly high drying potential, which is understood to mean low relative humidity and high temperature, compared with the state of the boundary layer of saturated air on the surface of the body.
In short wave infrared radiators of the prior art, the lamps are usually configurated in modules of 3 to 10 lamps each. These modules are mounted side by side, and thereby a drying zone extending all the way across the web is obtained. The density, or spacing, of the lamps is usually such that the power per unit area in the dryer varies between 100 and 450 kW/m.sup.2.
Dryers operating in the medium wavelength IR range are either electrically or gas-powered. In electrical apparatus, coiled Kanthal wire is made incandescent with the aid of electric current, either in a quartz tube or behind a ceramic brick. In the first instance the coiled wire serves directly as emitter, while in the second case the heat is first conductively transferred to the brick, whereafter the brick constitutes the emitter. In gas-driven systems a radiator, usually ceramic, is made incandescent with the aid of a flame, whereby the radiator becomes incandescent and then serves as emitter. Radiation is in part also emitted directly by the flame. As has been observed before, the maximum intensity wavelength of medium wave infras is 2.00-4.00 .mu.m, the corresponding radiator temperature being, as also has been observed, 1176.degree.C. to 450.degree. C. There is usually no need to cool the apparatus, nut whenever higher power density is aimed at, or when it is desired to enhance the ventilation and evaporation, the equipment is provided with ventilation systems. The maximum power density of medium wave infrared radiators varies, depending on method and temperature, between 40 and 100 kW/m.sup.2.
The drawbacks of short wave infraradiators include poor radiation efficiency in the wavelength range of the radiator, low overall efficiency, inefficient use of the cooling air towards drying, an expensive electrical control system, high operating costs, and fire hazard owing to high temperature.
The drawbacks of medium infraradiators include fairly low power per unit area whenever fast drying is aimed at, in the case of electric infras rather high operating costs, poor controllability, rather slow warming-up and after-glow, high fire hazard, explosion risk owing to gas handling, and inefficiency of the cooling air that is used, in view of the drying event proper.
Thus the significant deficiency of all existing infrared dryers, or IR dryers, made up of infrared radiators may be considered, when the dryer consists of short wave radiators, to be the inadequate efficiency of short wave radiators, resulting from the low coefficient of absorption in the radiator's wavelength range of the material to be dried. When the IR dryer consists of medium wave infrared radiators, one may name as a particular deficiency their inadequate controllability, owing to the prolonged afterglow of medium wave radiators. In addition, medium wave radiators yield only low rates of power per unit area.